A photovoltaic solar cell is formed of a planar semiconductor element in which, by means of incident electromagnetic radiation, generation of electron-hole pairs is obtained and charge carrier separation takes place at least one pn junction, such that an electrical potential difference arises between at least two electrical contacts of the solar cell and electrical power can be tapped off from the solar cell via an external electric circuit connected to said contacts.
In this case, the charge carriers are collected via metallic contact structures, such that, by making contact with said contact structures at one or more contact points, the charge carriers can be fed into the external electric circuit.
Competing requirements arise with regard to the design of the metallic contact structure: On the one hand, the average path length of a charge carrier in the semiconductor substrate to the closest contact point with a metallic contact structure is intended to be small so that ohmic losses on account of conduction resistances within the semiconductor are kept small. On the other hand, the contact area between metallic contact structure and semiconductor substrate is intended to be small since a high recombination rate is present at the contact area, compared with electrically passivated surfaces of the semiconductor substrate.
Particularly in the case of solar cells in which an emitter, and thus also the pn junction separating the charge carrier pairs generated, is formed at or in the region of the front side of the solar cell, said front side being designed for light incidence, electrical contact is made with the base typically by means of a metallic contact-making layer which is arranged on the rear side ad which is electrically conductively connected to the semiconductor substrate. In order to obtain high efficiencies, an efficient rear-side passivation, i.e. the obtaining of a low surface recombination rate for minority charge carriers, in particular in the region of the rear-side surface of the semiconductor substrate, and a formation of the electrical contact with a low contact resistance are essential in this case.
Thus, solar cell structures are known in which the rear side of the semiconductor substrate is covered substantially over the whole area with a passivation layer or insulating layer embodied as a silicon nitride layer, silicon dioxide layer or aluminum oxide layer or layer system, such that low surface recombination rates are obtained. It is only at point contacts that the passivation layer is opened in a linear fashion over a small area or in a point-like fashion and there is an electrically conductive connection to a metallic contact-making layer arranged on the passivation layer. In this case, the total area of the distributed contacts is significantly smaller than the area of the rear side of the solar cell. One such solar cell structure is, for example, the PERL structure (passivated emitter, rear locally defused) as described in J. Benick, B. Hoex, G. Dingemans, A. Richter, M. Hermle, and S. W. Glunz “High-efficiency n-type silicon solar cells with front side boron emitter,” in Proceedings of the 24th European Photovoltaic Solar Energy Conference (Hamburg, Germany), 2009 and in Zhao et al., Proc. of the 21st IEEE PVSC, 333 (1990). This structure, which makes possible rear-side contact-making in order to achieve high efficiencies, requires additional photolithography steps during production, such that an industrial implementation of said solar cell structure is not practicable or is at least highly cost-intensive.